Audio Conferencing in Office 365 is an add-on service that allows users call into meetings from their personal phones. Audio Conferencing allows up to 250 phone attendees at maximum. This service can be used with both Skype for Business and Microsoft teams.
You only need to set up Audio Conferencing for people who plan to schedule or lead meetings. Meeting attendees who dial in don't need any licenses assigned to them or other setup to access the conference.